Meter provers

ABSTRACT

A meter prover employs a piston arranged to slide in a bore within a proving cylinder which is coaxially disposed within a surrounding cylindrical shell. The effective area of the upstream face of the piston which can be acted on by fluid pressure is less than the effective area of the downstream face of the piston. In a proving run, fluid flow to the meter to be proved is diverted to an inlet in the shell upon closure of a bypass valve. The diverted fluid then flows through an annular space between the proving cylinder and the shell, enters the upstream end of the bore through openings in the cylinder, and drives the piston downstream along the cylinder in synchronism with the flow. Upon reopening of the bypass valve, equal fluid pressures act on the upstream and downstream faces of the piston and the difference in effective area of those faces causes the piston to return to the position it had at the start of the proving run.

FIELD OF THE INVENTION

This invention relates to meter provers for use, for example, in provingrotary flow meters of the type producing an electrical output pulse perincrement of rotation. The number of pulses per unit volume is acharacteristic of the meter which is defined as the k factor and it isthe purpose of the prover to enable calibration of the meter k factor. Aprincipal application is the measurement of oil flow rates and it ishere a requirement for the meter k factor to be determined to anaccuracy of at least 0.02%.

BACKGROUND OF THE INVENTION

The conventional form of meter prover utilised the passage of a spherealong an accurately dimensioned pipe between fixed detectors to displacea known volume of fluid. The volume of fluid displaced by the proverpasses in series through the meter to be proved and the number of pulsesgenerated in the meter during the passage of the sphere between thedetectors is counted to enable determination of the k factor. To achievethe necessary high accuracy, a large displaced volume is essential andprovers of 20 meters in length were not uncommon. Provers of this sizeare clearly impractical on oil rigs and other confined spaces andefforts have therefore been made to develop so called compact provers.By increasing the inherent accuracy of the proving operation with theuse of electrical pulse handling techniques, it has been possible toreduce the displaced volume necessary for the desired 0.02% accuracy andmeter provers have been produced in the form of a piston and cylinderhaving a stroke in the order of one meter.

With meter provers of the piston type, the difficulty is encountered ofhow to restore the piston to its start position after the end of aproving stroke. Various mechanisms have been suggested for this purposeincluding cable and winch arrangements (such as for example shown in FRNo. 2 471 590) or separate retraction pistons operating on compressedgas (see for example GB No. 2 023 295). Any such additional retractionmechanism requires to be carefully controlled so as to offer noimpedance whatever to the proving stroke but to restore the piston fullyand reliably to its start position in time for the next proving stroke.These meter provers have therefore been relatively expensive. Moreover,the retraction mechanism represents a further source of possiblemalfunction or breakdown.

Other meter provers restore the piston by reversing the flow through thecylinder, thus offering the possibility of proving in both directionsmovement of the piston. Provers of this sort are referred to asbidirectional in contrast to the unidirectional provers with separateretraction mechanisms described above. If a bidirectional prover is tobe connected in line with a working meter, it is necessary to provide afour way valve such that flow can be directed in opposite senses throughthe prover and in a by-pass mode around the prover. Care must be takento monitor any possible leakage in such four way valves using, forexample, block and bleed techniques. Four way valves with block andbleed facilities are, however, relatively expensive. There is a furtherdifficulty associated with bidirectional provers, namely that thedetection, control and processing systems have to be capable of dealingwith proving strokes in two opposite directions. This invariably adds tothe complexity and cost of the prover.

OBJECT OF THE INVENTION

It is an object of this invention to provide a meter prover which iscompact, reliable and relatively inexpensive.

The present invention consists in a meter prover for proving a meterconnected in a fluid flow path, comprising a proving bore; a pistonmovable along said bore in sealed engagement therewith; means forconnecting the proving bore in parallel with a section of said flow pathnot containing the meter and bypass valve means operable to cause flowselectively through the proving bore or through said parallel flow pathsection, the piston moving in a proving run in synchronism with flowalong the proving bore to establish a known displaced volume,characterised in that the effective area of the piston on the upstreamface thereof is less than the effective area on the downstream face sothat on commencement of flow through said parallel flow path sectionafter a proving run, substantially equal fluid pressure acting onopposite faces of the piston effects retraction thereof in a directionupstream.

Advantageously, the proving bore is defined by a proving cylinder lyingwithin a hollow cylindrical shell, the cylinder and shell beinginterconnected only at opposite axial extremes of the proving bore.

Preferably, the proving cylinder is formed with fluid inlet means andfluid outlet means adjacent respective opposite ends thereof, the shellbeing formed with fluid inlet means at an end thereof remote from thecylinder inlet means so that fluid flow through the prover passes alongan annular gap between the shell and cylinder before entering theproving bore. In this way, the exterior of the proving bore is exposedonly to fluid and not to the surroundings which may be at a differenttemperature. Moreover, the proving cylinder is not subjected to largedifferential fluid pressures and the necessary dimensional stability canbe achieved with a less substantial structure.

It is a general disadvantage of the piston type of prover that shouldjamming of the piston occur, the resulting fluid pressure transientsmight cause severe damage both to the prover and to the associated pipework. If a relief valve were to be provided in series with the meterprover, it would have--for the elimination of leaks--to be produced tothe same tolerances as the prover itself. Moreover, separate procedureswould have to be instituted to monitor leaks developing in use. Such arelief valve would accordingly be expensive to produce and would addfurther complications to the proving operation. It is therefore afurther object of one aspect of this invention to provide an improvedmeter prover which affords protection against the harmful consequencesof the piston jamming, without undue complication of the normal provingoperation.

Accordingly, the present invention consists in a further aspect in ameter prover comprising a cylinder having a proving bore and a pistonmounted in the cylinder for movement along the bore in sealed engagementtherewith, characterised in that the piston is formed in radially innerand outer parts which are separable in the axial direction, saidseparating movement being normally restrained by at least one elementwhich is adapted to undergo permanent deformation or fracture in theevent of fluid forces on the piston exceeding a predetermined threshold.Suitably, said element comprises a radially disposed shear pin.

With a meter prover according to this preferred form of the invention,any increase in fluid pressure caused by jamming of the piston wouldresult in shearing of the pin or pins holding the two piston partstogether so enabling the radially inner piston part to move with theflow away from the jammed outer part. Fluid will then flow through theradially outer part of the piston so preventing the fluid pressure fromrising to harmful levels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional elevation of part of the meter prover according tothis invention,

FIG. 2 is a detail view of part of FIG. 1,

FIGS. 3(a) to (d) are diagrams showing the installation and operation ofthe described meter prover,

FIG. 4 is a scrap view to an enlarged scale of part of FIG. 2,

FIG. 5 is a sectional elevation of a further part of the meter provershown in FIG. 1; the parts shown in FIGS. 1 and 5 being axiallycontiguous,

FIG. 6 is a section on 6--6 of FIG. 5,

FIG. 7 is a sectional view showing an alternative form of piston for usein the prover shown in FIG. 1, and

FIG. 8 is a detail view showing a further modification.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 of the drawings, the meter prover comprises two endplates 10 and 12 between which extends a generally cylindrical shell 14.The shell 14 has integral flange ends 16 which are of the same diameteras the end plates and which are secured to the respective end plateswith a series of bolts spaced equiangularly around the periphery. Theshell 14 has a fluid inlet port 18 at a location close to the end plate12. Within this end plate 12, there is a central aperture 20 serving asa fluid outlet port.

Coaxially mounted within the shell 14 is a proving cylinder 22 having acentral section 24 of constant wall thickness and shaped upstream anddownstrean end portions 26 and 28 respectively. This cylinder abuts thetwo end plates and is located radially by means of two locating rings 30provided one at each end. The central section 24 of the cylinder definesan internal proving bore 32.

The upstream end portion 26 has an interior circumferential groovedefining a bypass chamber 34 which is illustrated to a larger scale inFIG. 2. Four apertures 36 equally spaced around the circumference of thecylinder end portion 26 communicate between the bypass chamber 34 andthe annular gap 38 between the shell 14 and the cylinder central section24. The opposite end portion 28 is formed with an interiorcircumferential groove of generally regular form providing a downstreambypass chamber 39.

A piston 40 is mounted for sliding movement along the cylinder and has apiston rod 42 of hollow cylindrical form extending through a centralaperture 44 of the end plate 10. The piston, shown in more detail inFIG. 2, comprises a bulbous piston rod head 46 formed with an outercollar 48 supporting a cylindrical piston wall 50 which is coaxial withthe piston rod. Downstream of the collar 48 (that is to say to the rightin FIG. 1) the piston wall is provided with a series of four apertures52 spaced equally about the circumference thereof. Upstream of thecollar 48 the piston wall has two further series of four apertures; theapertures 54 remote from the collar being of the same dimensions asapertures 52 and the apertures 56 adjacent the collar being of a reducedaxial dimension. As seen best in FIGS. 2 and 4, the piston wall 50 isformed with an annular recess 51 opposite the location of the supportingcollar 48. Inside this recess are positioned two O-ring seals 53, acentral seal spacer 54 of generally dumb-bell shaped section ensuringthat the two seals are maintained at opposite extremities of the recess.

In operation, the prover is connected as shown in FIG. 3. The flow lineis connected to the shell inlet port 18 through pipe work 60. Returnpipe work 62 connects the fluid outlet 20 with the meter to be provedand a valve 64 is positioned in a bypass 66 between pipework 60 andreturn pipe work 62. If appropriate in particular circumstances, theprover may be arranged downstream rather than upstream of the meter tobe proved. During normal working, the valve 64 is open and the piston isin the start position shown in FIG. 3. The flow resistance offered bythe open valve is less than that of the prover so that flow will passthrough the valve, although the prover will contain fluid at the linepressure and line temperature. At the start of a proving operation, thevalve 64 is closed, as shown in FIG. 3a, causing fluid to divert throughinlet port 18; along the annular gap between the shell 14 and theproving cylinder 22; through the apertures 36 and into the bypasschamber 34. In the start position of the piston shown in FIGS. 2 and 3a, the piston sealing rings 53 lie within the bypass chamber 34 so thatfluid is free to pass through the piston wall apertures 52, along theproving bore 32 and through the fluid outlet port 20. After steady stateconditions have been achieved, the piston is nudged (as will bedescribed) in a direction toward the proving bore 32 taking the pistonseals 53 out of the bypass chamber 34 and into engagement with theproving bore as shown in FIG. 3b. Once this engagement is made, thepiston will travel along the proving bore in synchronism with fluid flowthrough the prover.

During this proving stroke, the output impulses from the meter to beproved are compared with the changing position of the piston as measuredby detectors on the free end of the piston rod which will be describedlater.

At the completion of the proving stroke, shown in FIG. 3c, the pistonsealing rings 53 enter the downstream bypass chamber 39, so enablingimmediate fluid flow through the relatively narrow piston wall apertures56 to the bypass chamber. After further slight movement, flow is alsoenabled through the larger apertures 54. In this way, the increasedfluid pressure upstream of the piston is smoothly reduced and the pistoncomes to rest. The design of the piston and piston rod in generallyhollow form is such that the mass of the piston/piston rod assembly islow and not greatly different from the volume of fluid displaced. Thismeans that the inertia of the piston is relatively low and the pistoncan be brought to rest quickly.

To complete the sequence of proving operations, the valve 64 is openedwith the effect that fluid pressure at the prover inlet and outlets isequalised. The piston is nudged to the left (as shown in FIG. 1) untilthe seals 53 re-enter the proving bore. It will be recognised that theeffective area of the downstream (with respect to the direction of flowin the proving stroke) face of the piston exceeds that of the upstreamface by a margin corresponding to the cross sectional area of the pistonrod.

With equal fluid pressure on either side of the piston, this inequalityof effective area results in movement of the piston in the upstreamdirection. (See FIG. 3d). The retracting movement of the piston willcontinue, expelling fluid contained in the cylinder to the left of thepiston, until the piston reaches the start position shown in FIG. 3.

By this means, retraction of the piston is achieved automatically andwith no separate retraction mechanism which would add costs andintroduce a further potential cause of malfunction or breakdown.

Further aspects of the apparatus according to a preferred form of thisinvention will now be described.

In order to ensure that there is no leakage past the piston seals 53 ablock and bleed technique is employed. It has previously been suggested(see for example FR No. 2 481 449) that the fluid pressure in theannular space between piston sealing rings should be monitored to detectleaks. This has involved, however, detecting changes which are verysmall in comparison with the absolute pressure measured and the limit offluid leakages that known arrangements can detect is capable ofimprovement. To this end, and with particular reference to FIG. 2, thepiston rod head 46 contains a differential fluid pressure transducer 70having a first input 72 connected through tube 74 with a radial bore 76which extends through the collar 48 and opens into the piston wallrecess 51. The above described seal spacer 54 has a series of radialapertures 55 (shown in FIG. 4) providing communication between thisradial bore 76 and the volume between the piston seals. A second inlet75 of the pressure transducer is connected through tube 81 with a smallaperture 82 in the piston rod head upstream of the collar 48. In thisway, the pressure transducer is capable of measuring a differentialpressure between the fluid in the proving bore upstream of the pistonand the fluid contained between the two sealing rings. In order toensure that the latter fluid is at a pressure measuably in excess of thepressure in the proving bore, the bypass chamber 34 is provided as shownin FIG. 4 with a slight chamfer 80 leading into the proving bore itself.On commencement of a proving run, the first of the piston rings 53enters the chamfer 80 and is compressed to its sealing configuration. Ata finite interval thereafter, the second piston ring, still in itsrelaxed state, makes preliminary engagement with the chamfer andeffectively traps a volume of fluid between the two sealing rings.Further movement of the piston takes the second piston ring through thechamfer effecting radial compression of the second sealing ring. Thefluid originally trapped in the chamfer has now to occupy a volume ofreduced radial dimension. The piston sealing rings have an inherentelasticity and will each be slightly deformed in the axially outwarddirection by this trapped fluid which will itself undergo some slightcompression. These phenomena lead to an increased pressure sensed at thefirst inlet of the differential pressure transducer. In this way, verysmall absolute changes in fluid pressure between the sealing ringsbecomes measurable so reducing further the limit of fluid leakage thatcan be detected.

With reference to FIGS. 5 and 6 there will now be described a mechanismfor nudging the piston into the proving bore and means for detecting theposition of the piston. A support disc 90 is mounted on fourequiangularly spaced pillars 92 which are parallel with the axis of theproving cylinder and which are secured at their respective opposite endsto an annular plate 94 bolted in turn to the end plate 10 of the provingshell. The support disc 90 has a central aperture 96 through which thepiston rod extends and is slidably supported. Two solid guide rods 98extend from the support disc 90, one on each side of the piston rod, andare joined at their free ends by tie plate 100. A carriage 102 issecured rigidly to the free end of the piston rod and has two laterallyextending wings 104. Each wing 104 is formed with an aperture 105through which the corresponding guide rod 98 extends so that thecarriage is free to slide along the guide rods. A cylindrical cover 107is mounted through integral flange 109 to the support disc 90 andextends over the length of the guide rods terminating in cover plate111.

In addition to the guide rods 98, there extend between the support disc90 and tie plate 100 a lead screw 106 and a nudge cylinder support bar108 of square cross section. The lead screw 106 is freely rotatablymounted and a bush 109 mounted on one lateral wing of the carriage 102may selectively be brought into threaded engagement with the lead screw.In the normal, disengaged position, the bush 109 slides freely relativeto the lead screw 106. In the engaged position, the carriage, piston rodand piston may be driven manually along the proving bore by rotation ofthe lead screw. This enables calibration of the prover in the field.

Intermediate its length, the nudge cylinder mounting bar 108 carriesfour brackets 110 (two only of which are seen in FIG. 5) to which aremounted a double acting nudge cylinder 112. Nudge piston rods 114 extendin opposite directions from the nudge cylinder 112, each rod terminatingat a nudge finger 116 mounted--through an integral bearing sleeve118--for sliding movement along the mounting bar 108. Beneath the bar108, each nudge finger is shaped as a yoke for engagement with thecarriage 102.

It will be seen that with the proving piston in its start position, thecarriage 102 will be engaged with left hand nudge finger 116. Actuationof the nudge cylinder causing the piston rods thereof to move to theright, will thus be effective to launch the piston on a proving run. Atthe end of the proving run, the carriage will come into engagement withright hand nudge finger and the nudge cylinder is preferably arranged toapply a decelerating force through the carriage to the piston. With thebypass valve moved to the open position, actuation of the nudge cylinderto move the piston rods thereof to the left, will bring the piston intothe proving bore, enabling the unbalanced fluid forces to effectretraction of the piston as described above.

Beneath the guide rods 98, and parallel to them, a linear measurementtape 120 extends between anchorage points 122 and 124 in the tie plate100 and support disc 90, respectively. This tape is of well known formand contains magnetic markings which can be detected by a lengthtransducer 126 mounted on the carriage 102 so as to overlie the tape.Signals from this length transducer can be interpreted to define theaxial position of the piston at any stage during the piston run. Thisability to measure the axial piston position continuously is importantas it enables the proving stroke to be defined by pulses from the meterundergoing proving. The proving stroke can thus be selected to occupy awhole number of meter revolutions, eliminating errors that would arisefrom cyclic variations in the meter.

A modified form of piston for use in the above described meter prover isillustrated in FIG. 7. The piston comprises an annular wall portion 200formed with spaced outer O-ring seals 202. Centrally within the piston,there is mounted an inner disc member 204 with sealing engagementbetween the disc member and the wall portion being ensured by a pair ofinner O-ring seals 206. The disc member 204 is formed integrally withthe piston rod 208 and channels 210, 212 cut in the rod and disc membercommunicate with the proving bore and--through a registering passage 214in the wall portion--with the volume between the outer O-ring seals 216.These passages enable block and bleed monitoring of possible leakagepast the outer O-ring seals 202 in the manner described above as well aspast the inner O-ring seals 206.

The disc member is secured to the piston wall by means of radiallydisposed shear pins 218. It will be appreciated that in normal use thepiston will function as an integral structure. However, in the event ofthe piston jamming within the proving bore, fluid forces acting on thedisc member 204 will cause the shear pins to fracture so enabling thedisc member, and integral piston rod, to separate axially from thejammed cylinder and continue along the bore with the fluid flow. In thiscondition, fluid can pass freely through the cylinder and around thedisplaced disc member. The cross-sectional area of the relief passagecreated in this manner should ideally be no less than thecross-sectional area of the pipe work leading to the meter prover.

In a further modification, the dividing line between the inner and outerradial parts of the piston takes the form not of a cylinder but afrusto-conical surface. The cone angle being such as to permitdownstream movement of the piston rod relative to the remainder of thepiston. In yet a further modification, the radially disposed shear pinscan be replaced by a solid shear ring of toroidal form, the shear ringthen functioning both as a seal and as an overload protection device.Alternatively, the shear ring may be replaced by an inflatable ring thematerial and inflation pressure of which are chosen so that the ringwill burst at the maximum safe fluid pressure within the proving bore.The time taken for the ring to burst at the threshold can be reduced bythe provision of a cutting ring or by the use of a plunger valve formedwith a needle point.

In a further modification of the described meter prover, the piston rodis secured to the piston not directly but through a flexible couplingcomprising, for example, a block of elastomeric material. By thisexpedient, slight movement of the piston relative to the piston rod canbe accommodated to prevent jamming.

In a further modification, the linear measurement device comprising atransducer on the piston rod end cooperable with a fixed tape, isreplaced by a fixed scale disposed coaxially of the proving bore andcooperable with a transducer mounted on the piston itself. With such anarrangement it is possible for the rigid piston rod to be replaced bybellows type or telescopic piston extension means extending from theupstream face of the piston to the corresponding end of the provingbore. In this way, it is ensured that the effective area of the upstreampiston face is less than that of the downstream face so that the abovedescribed automatic retraction of the piston is still achieved. By"effective area" is meant the component of area normal to the directionof movement of the piston.

A further modification is illustrated in FIG. 8. If the excess pressurecreated in the above described block and bleed arrangement is felt to beinsufficient, a plunger 300 can be slidably mounted in a sleeve 301projecting from the carriage 102. This plunger is depressed at the startof the proving stroke through cooperation with a cam surface on, forexample, the underside of the nudge cylinder mounting bar. A pressuresurge is developed in chamber 302 of a cylinder block 303; which surgeis communicated to the volume between the piston seals, along tubing304.

In other modifications, the proving bore is established by a structureother than the described arrangement of a right proving cylinder and acoaxial outer shell; the proving bore may for example be formed in ablock of rectangular or other non-cylindrical external shape. It hasbeen explained that there may be advantage in having a piston inradially inner and outer parts which are separable in the event ofjamming of the piston. There may be advantage, for other reasons, inhaving a piston in two axial parts. Thus, maintenance of the pistonsealing rings would be facilitated.

It will be apparent to the skilled man that still further modificationsto the described embodiments can be made without departing from thescope of this invention as set forth in the accompanying claims.

We claim:
 1. In a meter prover for proving a meter connected in a fluidflow path where the meter prover is of the type having(a) meansproviding a proving bore; (b) a piston disposed in the proving bore andmovable along said bore in sealed engagement with the wall of the bore,said piston starting its proving run from an initial upstream position;(c) means for connecting the proving bore in parallel with a section ofthe flow path not containing the meter; and (d) by pass valve meansoperable to cause fluid flow selectively through the proving bore orthrough said parallel flow path section;whereby in a proving run, thepiston moves axially in synchronism with fluid flow along the provingbore and displaces a known volume within the bore, the improvementwherein the effective area of the upstream face of the piston acted onby fluid pressure is less than the effective area of the piston'sdownstream face acted on by fluid pressure so that on reestablishment offlow through said parallel flow path section after a proving run,substantially equal fluid pressures act on said faces of the piston,whereby the difference in effective area of those faces causes thepiston to be restored to its initial upstream position.
 2. Theimprovement according to claim 1, whereinthe proving bore is terminatedat its upstream end by an end wall,and the improvement further includespiston extension means extending from the upstream face of the piston tothe upstream end wall of the proving bore.
 3. The improvement accordingto claim 2, whereinsaid extension means comprises a rigid piston rodattached to the piston and extending through the upstream end wall ofthe proving bore.
 4. The improvement according to claim 3, furtherincludinglength transducer means carried by said piston rod, and a scalefixed in position relative to said piston rod and cooperable with saidlength transducer means for enabling determination of the axial positionof the piston in the proving bore.
 5. The improvement according to claim1, whereinsaid proving bore is embodied in a proving cylinderand theimprovement further includes a hollow cylindrical shell surrounding theproving cylinder, the cylinder and shell being interconnected only atthe axial ends of the proving bore.
 6. The improvement according toclaim 5, whereinthe proving cylinder has a fluid inlet and a fluidoutlet adjacent respective opposite ends of that cylinder, and the shellhas a fluid inlet adjacent its end which is remote from the cylinderfluid inlet, whereby fluid flow through the prover passes along anannular gap between the shell and the cylinder before entering theproving bore.
 7. The improvement according to claim 1, whereintheproving bore is embodied in a hollow cylinder having a chamber at itsdownstream end whose diameter exceeds that of the bore, and the pistonhas an axial length exceeding that of the chamber and is provided withpassage means enabling flow through the piston between the chamber andthe bore when the piston is in a position where it extends across thechamber.
 8. The improvement according to claim 7, whereinthe piston hasan annular wall that is coaxial with the bore and the annular wall hasat least one passage which enables flow through the piston between thechamber and the bore when the piston's annular wall extends across thechamber.
 9. The improvement according to claim 1, whereinthe pistoncomprises radially inner and outer parts which are separable in theaxial direction, andthe improvement further comprises restraining meansfor restraining separation of those parts, the restraining meansenabling those parts to separate where the fluid forces on the pistonexceed a predetermined threshold.